When you go to buy sunscreen, you might look for one thing: SPF.
当你买防晒霜的时候，你可能会找一个指标：SPF。
It stands for sun protection factor, and based on that name, it seems like it should tell you how well you'll be protected from the sun.
它代表防晒指数，这个名字好像是告诉你产品的防晒效果如何。
Except… that's not totally true.
但这并不完全正确。
Because while SPF might sound like a perfect, rational scientific unit, there are a lot of flaws in the way it's named and the way it's tested.
虽然SPF听起来像是一个完美合理的科学单位，但是它的命名方式和试验方式有很多漏洞。
So if you want to be as protected from the sun as possible, you're going to have to know a lot more than that one number.
因此如果你想防晒效果更好，你需要知道更多。
First, as a disclaimer, it's worth noting that countries approach sunscreen regulations differently.
首先，作一个声明，值得注意的是，各国对防晒霜的规定有所不同。
This episode is a good general overview, but keep in mind that things might be different where you live.
本集只是给大家大概讲讲，但是要记住不同国家根据的标准有所不同。
Okay, that being said, onward.
好的，说到这里，我们继续。
Essentially, SPF measures how much radiation is blocked from reaching your skin.
实际上，SPF 衡量的是被阻隔在你皮肤外的辐射量。
So if it's SPF 30, it will protect you against thirty times more exposure than if you didn't wear sunscreen.
如果 SPF 为 30，那么与你不涂防晒霜时相比，涂防晒霜能阻挡 30 倍的辐射。
That translates to roughly 97% protection, and it explains why anything higher than SPF 30 isn't that much more effective.
这就表示防晒霜大约能起 97%的防晒作用，这就解释了为什么任何 SPF 值高于三十的防晒霜没有更明显的效果。
SPF 50, for example, blocks fifty times the radiation, or 98% of it.
再比如说 SPF 是 50 的话，可以阻挡 50 倍的辐射，或者是 98%的辐射。
Which isn't that different.
没什么太大差别。
Still, regardless of what that number says, SPF only tells you how protected you'll be from a specific type of radiation.
但是，不管 SPF 值是多少，它只能告诉你能免受多少特定类型的辐射。
The rating you see on sunscreen bottles is only required to show protection against UVB rays, the type of radiation that causes sunburns and leads to skin cancer.
你在防晒霜上看到的数值只显示了对 UVB 射线的防护效果，这种辐射会导致晒伤甚至引发皮肤癌。
It doesn't have to show protection against UVA radiation, which causes wrinkles, premature skin aging, and again, skin cancer.
但是不代表能够预防可以导致皮肤老化、皱纹、引发皮肤癌的 UNA 射线。
So even if you're using SPF one thousand, that doesn't mean you're protected against UVA.
因此，即使你用了 SPF1000 的防晒霜，也并不意味着能阻隔 UVA 的辐射。
If you want to avoid both kinds of rays, you'll need to look for broad-spectrum sunscreen.
如果你想要避免这两种辐射，你需要使用光谱防晒。
Another thing worth knowing about SPF is that the way it's tested isn't bulletproof.
你应该知道另外一件关于 SPF 的事情是它的测试方式并不是完美的。
To figure out how much UVB something blocks, you'd think scientists would use some sort of skin-like compound in a Petri dish.
为了算出有多少 UVB 会被阻挡，你可能认为科学家会在培养皿中用一些类似于皮肤的化合物。
Like, just shine some light on this stuff, and see how much radiation gets through.
直接把它们放在光下，然后看看有多少辐射穿过。
But while that is how they test broad-spectrum sunscreen, it isn't how they test SPF.
但这是他们测试光谱防晒霜的方法，而不是测试 SPF 的方法。
To get an SPF rating, researchers use volunteers.
研究人员通过志愿者测试来得到 SPF 的数值。
First, they shine a UV lamp on one spot on participants' bare backs to get a baseline for their tendency to burn.
首先，他们将紫外线灯照在参与者裸露后背的一个点上，得到一个晒伤临界点的基础值。
Then, a lab tech applies a gob of sunscreen to a different spot and allows it to dry.
然后，实验人员会在不同地方涂上防晒霜并且等它变干。
Finally, the tech turns the UV light back on, and sees how much longer the participants last before turning red again.
最后，技术人员再次打开紫外线灯，观察参与者的皮肤经过多久会再次变红。
If they burned in four minutes unprotected, but 60 minutes with sunscreen, that earns an SPF fifteen rating.
志愿者在没涂防晒霜时，四分钟就会晒伤，但是涂了防晒霜之后，可以坚持 60 分钟，那么得到的 SPF 值为 15。
And if this process seems sketchy and full of issues… yes.
这个过程是不是看上去很粗略而且有很多问题啊……是的。
Yes, it does.
你说对了。
For one, people turn red at different amounts of exposure.
首先，人的皮肤在不同程度的暴露下变红程度会有差别。
So if you used a bunch of pale people in one study and a bunch of people with darker skin in another, you'd get totally different results.
所以如果用一组皮肤相对比较白的人做实验，再用一组皮肤比较黑的人做实验，会得到完全不同的结果。
Then there's the amount of sunscreen used in these tests.
此外，这些测试中试用了大量的防晒霜。
While there's tons of variability between testers, protocol calls for no less than two milligrams of sunscreen per square centimeter of skin and that's significantly more than people tend to use in real life.
虽然测试者之间的差别很大，但是实验方案要求每平方厘米皮肤上，至少要有两毫克的防晒霜，这比人们在现实生活中使用的要多得多。
In one study, it was more than twice the amount.
在一项研究中，防晒霜的用量超过了日常的两倍。
Ultimately, these tests just aren't representative of what happens in the real world.
所以这些测试并不能代表真实情况。
They don't take into account things like skin color, or how much sunscreen people actually use.
他们没有考虑到肤色差异，或者是人们涂抹防晒霜的实际用量。
So when you're buying this stuff, it's probably worth keeping those things in mind.
因此，在你买防晒霜的时候，需要考虑一下以上因素。
For example, maybe this means it's worth putting on sunscreen more often than you currently do.
例如，你可能需要更频繁地涂抹防晒霜。
If nothing else, though, the good news is that researchers are at least aware that there's a problem here, and they've been working on protocols to hopefully get more consistent results.
别的不说，好消息是研究人员至少意识到了存在的问题，并且一直在改进实验方案，希望能够得到更加一致的结果。
Their ideas include using robots that spread sunscreen evenly in all conditions and shifting towards standardized in vitro testing using chemicals in a dish—no human skin involved.
其中包括使用机械设备，在各种情况下均匀涂抹防晒霜，在不使用人类皮肤的情况下，用培养皿中的化学物质进行标准化的体外测试。
This will let scientists get way more data points and will improve accuracy.
这将使科学家获得更多的数据，也可以提高准确性。
At the end of the day, SPF can be a helpful way to understand what you're buying.
不管怎么说，SPF 可以帮助你了解你要买的防晒霜。
To make an informed decision, you just need to know what it says and where those numbers are coming from.
要做出正确的决定的话，你只需要知道 SPF 是什么意思，以及这些数字来自于哪里。
And it's also worth knowing that not all hope is lost, either.
同样要知道的是，它并不是一无是处。
Even if this is a bit of a flawed system, multiple studies have shown that consistent use of sunscreen, even at low SPF, reduces your risk of skin cancer.
即使这是一个有缺陷的方法，但多项研究表明，持续使用防晒霜，甚至是低 SPF 的防晒霜，也能降低患皮肤癌的风险。
So, it's always worth lathering up!
所以，防晒霜总是值得用一下的！
If you've ever wished you just didn't have to wear sunscreen—well, I've got some sad news for you.
如果你曾希望自己不用涂防晒霜，那么我有一些坏消息要告诉你。
Almost all amphibians, reptiles, birds, and fish can make their own sunscreen.
几乎所有的两栖动物、爬行动物、鸟类和鱼类都能自己分泌“防晒霜”。
Mammals are kind of the odd ones out.
哺乳动物是个例外。
If you want to learn more, you can watch our episode about the missing sunscreen gene after this.
如果你想知道更多，可以看我们关于防晒基因消失的视频。
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